1. Field of the Invention
This invention relates generally to disk drives, and more particularly, to methods of monitoring the operation of disk drives.
2. Description of the Related Art
Disk drives (e.g., hard disk drives) typically utilize a plurality of heads in proximity to magnetic media on a plurality of rotating disks. Each head has a read transducer for reading data from the magnetic media. Each head may also have a write transducer including a pole tip for writing data to the magnetic media.
Generally, higher data storage densities on the magnetic media are obtained by having the read and write transducers close to the magnetic media without touching the magnetic media. The distance between the rotating disk and one or both transducers (referred to as the “flying height” of the head) is typically quite small (e.g., less than one microinch). It is desirable to maintain the flying height at a predetermined level while avoiding contact between the head and the rotating disk. For example, dynamic flying height (DFH) disk drives use an internal heater to heat the head to control protrusion by thermal expansion of or around the transducers (and hence the flying height) of each head. In this way, the desired head-media spacing for each head in the whole population of heads can be maintained.
Various techniques have been used previously to monitor or detect the flying height of the head over the rotating disk. In “Head-Disk Dynamics in the Flying, Near Contact, and Contact Regimes,” J. Tribology, July 2001, Vol. 123, Issue 3, pages 561-565, R.-H. Wang et al. describes the dynamics of flying near contact, explaining that there is an increase in the spacing modulation as the head gets closer to the rotating disk. As the head gets closer to the disk, the air bearing and the slider begin to resonate at certain frequencies. These vibrations cause variations in the spacing between the head and the magnetic media that result in modulation of the signal read from the magnetic media by the read transducer (referred to as the read-back signal). Wang et al. also discloses measuring the spacing modulation using a laser Doppler vibrometer (LDV) which measures motion of the head by analyzing laser light reflected from the head. Analysis of the read-back signal based on the Wallace equation to measure head/media clearance has also been described by U.S. Pat. Nos. 4,777,544 and 5,130,866. Detection of contact between the head and the rotating disk by analyzing frequency jitter in the read-back signal due to mechanical vibrations of the head along the direction of the track (which is parallel to the rotating disk surface) has been described by U.S. Pat. No. 5,594,595.
Laser doppler vibrometry typically utilizes relatively expensive equipment and requires that the disk drive be opened sufficiently to allow a laser beam to reflect from the head. Each of these factors makes laser doppler vibrometry inconvenient for production-line characterization of disk drives. In addition, analysis of the read-back signal to detect frequency modulation would be difficult for a digital system since the carrier frequency of the read-back signal is in the range of hundreds of MHz and such high sampling rates would be required.